To insert such a screw anchor into a support made of friable material, to continue with this example, all that is necessary is for the screw anchor to be screwed into the support without it being necessary beforehand to drill a hole in the support, as is often needed with numerous other screw anchors. By virtue of this, fitting is greatly simplified.
Once the screw anchor has been anchored in the support, in order to fix a piece to the support, either a fastener is introduced into the bore of the body of the screw anchor, of the screw anchor body is hollow and pierced with such a bore, or a nut is screwed onto the threaded shank portion that extends the screw anchor body beyond its bearing flange, if such is the case.
A screw anchor of the type defined hereinabove is known from document EP 0165 674.
In one of the screw anchors of this document, the drilling end consists of a flat blade of a certain length with, at its free end, drilling teeth, the utility of such an arrangement being that the actual drilling operation is more or less completed before the external screw thread of the screw anchor begins to tap into the support material.
However, it sometimes happens that support panels, even those made of friable material, are slightly too hard for such screw anchors and therefore give rise to the risk either of breaking the drilling blade or of making it impossible to anchor the screw anchors.
U.S. Pat. No. 6,354,779 teaches a screw• anchor for friable material, comprising a roughly cylindrical body with a drilling portion provided, at its free end, with drilling teeth, at the other end of the body, a bearing flange and an external screw thread wound around the body in one direction, the drilling portion being configured as a portion of a drill bit.
However, the central tooth of the drill bit portion looks like a centering pointed cone, without any cutting edge, so that the anchor of this document, since the central tooth just pushes the material, has the drawback of a risk to blow up the panel.